THAT1206 Datasheet, PDF (9/10 Pages) List of Unclassifed Manufacturers – InGenius High-CMRR Balanced Input Line Receiver

600033 Rev 0A
Preliminary Information
Page 9
common-mode input impedances are inextricably
linked, and of similar magnitude, it is not possible to
increase common-mode input impedance without
compromising noise performance.
3-op amp instrumentation amplifier
This topology, shown in Figure 11, was also dis-
cussed earlier. It relies on input buffers OA1 and
OA2 to raise the common-mode and differential-
mode input impedances. The following diff amp,
OA3 (which can be of the precision 4-resistor op amp
type), is then used to reject the common-mode signal
while extracting the differential signal.
This approach will require reasonably low values
for Ri1 and Ri2 (< 100 kW or so) unless the OA1 and
OA2 use FETs at their inputs. This would limit the
common-mode input impedance to a few hundred
kilohms.
If FET-input devices are used for OA1 and OA2,
Ri1 and Ri2 can be made quite large — on the order
of 10 megohms. Unlike the resistors in the conven-
tional diff amp stage, these resistors will be shunted
by the driving source impedance, and so contribute
negligible noise.
At first glance, this might seem to be an excellent
solution. However, there are disadvantages to this
approach. First, the designer must select a FET-
input op amp that is low-noise and that exhibits no
phase inversion (sign reversal) with large differential-
and common-mode signal swings. This, of course,
results in a cost penalty that is somewhat exacer-
bated by the price premium for high-value resistors.
Second, this design requires at least two IC pack-
ages — a dual FET op amp and the precision input
stage.
Third, while the large-value input resistances are
shunted when there is a source connected to the
input, there is no guarantee that long cables will al-
ways be properly terminated. With an unterminated
cable plugged into the associated XLR jack, Ri1 and
Ri2 are no longer shunted and become not only large
noise sources themselves, but will do little to reduce
pickup on the cable.
The THAT 1200-series input stages avoid these
problems altogether. They exhibit high common-
mode input impedance as a result of their boot-
strapped topology, while maintaining reasonable dif-
ferential input resistances that can be left unshunted
with no fear of stray pickup or excessive noise contri-
bution.
Transformers
When true electrical isolation is required, a trans-
former may be the only solution. Transformers suit-
able for pro audio, however, tend to be costly and
take up valuable board real estate. In addition, some
transformers can color the sound in ways that elec-
tronic solutions do not.
Fortunately, it is usually not the case that galvanic
isolation is required, and in most cases it is the
common-mode signal rejection properties of a trans-
former that is sought after. By providing the high
common-mode input impedance of a transformer
with the size and cost of an 8-pin integrated circuit,
the THAT 1200-series provides designers with an al-
ternative that provides excellent interference rejection
in real-world applications.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com